Recently, a mobile communication apparatus such as a portable telephone has been down-sized and devised to operate at lower voltages with reduced power consumption. With this progress, intensive investigation has been made to develop monolithic elements which can be mounted in a portable apparatus. However, since a bandpass filter and a duplexer are bigger in size than other high frequency components, there is little advantage to fabricate these elements monolithically together with other elements. Moreover, it is very difficult to fabricate a power amplifier as a monolithic element. For this reason, a duplexer, a power amplifier, a bandpass filter, a low noise amplifier arranged upstream of the bandpass filter, etc. have been developed as respective discrete elements and fabricated as respective modules. When these discrete elements are fabricated as modules, wiring for connecting a plurality of parts and circuitry for matching impedance are formed, and therefore the discrete elements as units are very large in size.
On the other hand, there have been made various studies on amplification of surface acoustic waves. In order to amplify the surface acoustic wave, it is known to propagate the surface acoustic wave in a surface of a piezoelectric substrate and couple the electric field generated by the wave with carriers in a semiconductor. Actual surface acoustic wave amplifiers are classified into three types according to the types of combination of the piezoelectric material for propagating the surface acoustic wave and the semiconductor: (1) a direct type amplifier (FIG. 3); (2) a separation type amplifier (FIG. 4); and (3) a monolithic type amplifier (FIG. 5). As shown in FIG. 3, the direct type amplifier is an amplifier having the structure which has a substrate 7 composed of a material, such as CdS or GaAs, with both piezoelectric characteristics and semiconductor characteristics simultaneously, on which input and output electrodes 4 and 5 are provided, with the substrate 7 being sandwiched by electrodes 6 for applying a direct current electric field to the substrate 7. However, a piezoelectric semiconductor with large piezoelectric properties and large mobility has not been found so far. As shown in FIG. 4, the separation type amplifier is an amplifier having the structure in which a semiconductor 3' of a large mobility is disposed on a piezoelectric substrate 1 of large piezoelectric property with a gap 8. Input and output electrodes 4 and 5 are provided on the substrate 1, and electrodes 6 for applying a direct current electric field to the semiconductor 3' are provided on both sides of the semiconductor 3'. In the amplifier of this type, surface flatness of the semiconductors and the piezoelectric substrate and the size of the gap 8 have a great effect on the amplification gain. In order to obtain a practically acceptable amplification gain, the gap 8 must be made as small as possible and maintained constant over an operation range and so that industrial fabrication of the amplifiers with such a gap is very difficult. As shown in FIG. 5, the monolithic type amplifier is an amplifier having the structure in which a semiconductor 3' is formed on a piezoelectric substrate 1 via a dielectric film 9 without a gap. Input and output electrodes 4 and 5 are provided on the piezoelectric substrate 1, and electrodes 6 for applying a direct current electric field to the semiconductor layer 3' are provided on both sides of the semiconductor layer 3'. The monolithic type amplifier can achieve a high gain and be used in a high frequency region. Moreover, the monolithic type amplifier is said to be suitable for mass production. However, application of these surface acoustic wave amplifiers to a mobile communication apparatus such as a portable telephone has not been studied yet.
In order to realize a monolithic type amplifier, a semiconductor film of good electric characteristics must be formed on a piezoelectric substrate and the semiconductor film must be sufficiently thin so that there can occur efficient interaction between the surface acoustic wave and the carriers in the semiconductor. According to the study by Yamanouchi et al. of Tohoku University in 1970s (Yamanouchi K., et, al., Proceedings of the IEEE, 75, p726 (1975)), an electron mobility of InSb of 1,600 cm.sup.2 /Vsec was achieved using the structure in which SiO is coated on a LiNbO.sub.3 substrate to a thickness of 30 nm and then InSb thin film is evaporation-deposited on the substrate to a thickness of 50 nm. When a DC voltage of 1,100 V was applied to a surface acoustic wave amplifier having the semiconductor films, an amplification gain of net gain 40 dB was obtained at a center frequency of 195 MHz. Furthermore, based on their theoretical calculation, Yamanouchi et al. predicted that in an InSb thin film of 50 nm thick, the maximum electron mobility is 3,000 cm.sup.2 /Vsec because of surface scattering of carriers (Yamanouchi et al., Shingaku Gihou, US78-17. CPM78-26, p19 (1978)). That is, the monolithic type amplifier faces the trouble that a thin film semiconductor layer having good electric characteristics is difficult to be formed on a piezoelectric substrate. Moreover, a conventional structure requires a dielectric film such as SiO in order to prevent deterioration of InSb and a LiNbO.sub.3 substrate because of diffusion of oxygen from the LiNbO.sub.3 substrate. Moreover, when a surface acoustic wave amplifier is used as an amplifier of a high frequency portion of a portable apparatus and a bandpass filter, the surface acoustic wave amplifier is useless if it gives no amplifying effect at a driving voltage of 3 to 6 V. A conventional monolithic amplifier needs a high voltage and there was no surface acoustic wave amplifier that could be driven at low voltages. Furthermore, there is the problem that a surface acoustic wave convolver, which makes use of interaction between a surface acoustic wave and electrons like the surface acoustic wave amplifier, gives an insufficient gain.
In general, an amplification gain, G, of a surface acoustic wave amplifier is given by the following equation: ##EQU1##
where A=constant, k.sup.2 =an electromechanical coupling coefficient, .epsilon.p=an equivalent dielectric constant of a piezoelectric substrate, .sigma.=a conductivity, h=a film thickness of an active layer, .mu.=an electron mobility, E=an applied electric field, and v=a velocity of a surface acoustic wave. In order to obtain a large amplification gain at a low voltage of a practical level, it is necessary that; (1) a semiconductor thin film is formed which has a high electron mobility and whose film thickness is as thin as possible; and that (2) a piezoelectric substrate is used whose k.sup.2 is as large as possible.
The present inventors have made intensive investigation on the above problems and as a result found that an active layer which is a thin film and has good electric characteristics can be obtained by inserting a buffer layer between the piezoelectric substrate and the active layer. The present inventors also found that an electromechanical coupling coefficient k.sup.2 of the piezoelectric substrate far larger than that of a bulk can be achieved by using a multilayer piezoelectric thin film substrate of at least three layers. Furthermore, the present inventors confirmed that a surface acoustic wave amplifier is fabricated using the semiconductor layer or the piezoelectric thin film substrate and that good amplification gains can be obtained at practical low voltages by this amplifier, thus accomplishing the present invention. In addition, an electron mobility of 5,000 cm.sup.2 /Vsec or more of the active layer has been achieved with the semiconductor film structure of the present invention. That is, the present invention provides a surface acoustic wave functional element comprising a piezoelectric substrate, input and output electrodes provided on the piezoelectric substrate, semiconductor layers provided between the input and output electrodes. The semiconductor layers include an active layer and a buffer layer lattice matched to the active layer. Here, by the term "active layer" is meant a layer which oscillates a surface acoustic wave which is being propagated with energy of carriers in the semiconductor.
In the present invention, the thin film active layer can have very good electric characteristics of an active layer because the crystallinity of the active layer can be improved by inserting a buffer layer between the piezoelectric substrate and the active layer. Moreover, the present inventors found that when the lattice constant of the crystal forming the active layer is made equal or similar to that of crystal forming the buffer layer, the crystallinity of the active layer can be further improved, and that the electric characteristics of the active layer can be markedly improved even when the active layer is in the form of a thin film. The present inventors also found that still better electric characteristics can be obtained by using a compound semiconductor containing Sb as the buffer layer of the present invention. The buffer layer of the present invention is characterized by high resistance and-small attenuation of a surface acoustic wave therein. The buffer layer of the present invention has superior properties in that it prevents the active layer from being deteriorated by oxygen from the piezoelectric substrate even when no dielectric film such as SiO is provided on the piezoelectric substrate and that it grows at low temperatures so that it does not deteriorate the piezoelectric substrate.
The piezoelectric substrate of the present invention comprises a multilayer piezoelectric body having at least three thin film layers, the layers having at least two different electromechanical coupling coefficients. Among the layers of the multilayer piezoelectric body, a central layer thereof has the largest electromechanical coupling coefficient. This facilitates efficient concentration of energy of the surface acoustic wave on a surface, so that the electromechanical coupling coefficient is made to be far larger than that of each piezoelectric body constituting each layer.
With the surface acoustic wave amplifier using the semiconductor layer of the present invention, the amplifying effect can be achieved at practical low voltages at which a portable or device is used. Moreover, a far larger amplification gain can be achieved using the multilayer piezoelectric body of the present invention.
Furthermore, with respect to the surface acoustic wave convolver, the interaction between the surface acoustic wave convoluted and electrons is strengthened because of a high electron mobility of the semiconductor layers, resulting in a gain larger than that of a conventional structure.
Furthermore, when the surface acoustic wave functional element of the present invention, which has a large amplification gain at practical low voltages, is used as a device for (i) a bandpass filter and a low noise amplifier, (ii) a bandpass filter and a power amplifier, or (iii) a bandpass filter, amplifiers, and a duplexer, in a mobile communication apparatus, the mobile communication apparatus can be markedly down-sized, thinned and lightened in weight. Therefore, the transmitting/receiving circuit of a mobile communication apparatus such as a portable telephone or a cordless telephone falls within the range of the present invention where the surface acoustic wave functional element having the high amplification gain is formed as an amplifier and a bandpass filter, or an amplifier, a bandpass filter, and a duplexer.
The present invention will be described in detail below. FIGS. 1A and 1B illustrate a basic surface acoustic wave functional element of the present invention. FIG. 1A is a cross sectional view showing a surface acoustic wave functional element of the present invention, and FIG. 1B is a perspective view showing the surface acoustic wave functional element of the present invention. Reference numerals 1, 2, 3, 4 and 5 designate a piezoelectric substrate, a buffer layer, an active layer, an input electrode and an output electrode, respectively.
According to the present invention, on the piezoelectric substrate 1 are arranged the input and output electrodes 4 and 5 at a distance from each other between which the active layer 3 is formed on the piezoelectric substrate 1 via the buffer layer 2.
In the present invention, the piezoelectric substrate 1 may be a piezoelectric single crystal substrate, or a piezoelectric thin film substrate. For the piezoelectric single crystal substrate, an oxide-based piezoelectric substrate is preferable. For example, LiNbO.sub.3, LiTaO.sub.3, or Li.sub.2 B.sub.4 O.sub.7 is preferably used. Moreover, a substrate cut surface of LiNbO.sub.3 of 64.degree. Y cut, 41.degree. Y cut, or 128.degree. Y cut, or Y cut, or LiTaO.sub.3 of 36.degree. Y cut can be used preferably. The piezoelectric thin film substrate has the structure in which a piezoelectric thin film is formed on a single crystal substrate of sapphire or Si, etc. Preferred thin film materials for the piezoelectric thin film include, for example, ZnO, LiNbO.sub.3, LiTaO.sub.3, PZT, PbTiO.sub.3, BaTiO.sub.3 or Li.sub.2 B.sub.4 O.sub.7. Furthermore, a dielectric film such as SiO, SiO.sub.2, etc. can be inserted between a Si substrate and the above piezoelectric thin film. As the piezoelectric thin film substrate, a multilayer film can be formed which is fabricated by growing the above piezoelectric thin films of different types one above the other on the single crystal substrate of sapphire, Si, etc.
When the piezoelectric substrate 1 of the present invention comprises a multilayer piezoelectric element of at least three layers having at least two different electromechanical coupling coefficients and in which a piezoelectric film located in a central portion of the multilayer piezoelectric body has the largest electromechanical coupling coefficient, large electromechanical coupling coefficients can be obtained.
With respect to the multilayer piezoelectric substrate, an example of the three-layer structure will be described in detail below referring to FIG. 2. The multilayer piezoelectric substrate 20 of the present invention has the structure in which there are provided on a piezoelectric substrate 21 a first piezoelectric film 22 and a second piezoelectric film 23. Here, the electromechanical coupling coefficients of the piezoelectric substrate 21, the first piezoelectric film 22 and the second piezoelectric film 23 are assumed to be k, k.sub.1, and k.sub.2, respectively, and the velocities of Rayleigh waves of the piezoelectric substrate 21, the first piezoelectric film 22 and the second piezoelectric film 23 are assumed to be V, V.sub.1 and V.sub.2, respectively. The film thicknesses of the first and second piezoelectric films 22 and 23 are assumed to be h.sub.1 and h.sub.2, respectively. Then, it is necessary that k.sub.1 is larger than k and k.sub.2, and preferably k.sub.1 is larger than k and k.sub.2 by a factor of 1.2 or more, more preferably by a factor of 2 or more. Moreover, when k.sub.1 is greater than k and k.sub.2 and V.sub.1 is greater than V and V.sub.2, a far larger electromechanical coupling coefficient can be obtained. V.sub.1 is preferably larger than V and V.sub.2 by 100 m/s, and more preferably by 250 m/s. Moreover, h.sub.1 is normally equal to or more than 30 nm and equal to or less than 20 .mu.m, and more preferably equal to or more than 80 nm and equal to or less than 5 .mu.m, far more preferably equal to or more than 100 nm and equal to or less than 2 .mu.m. In general, h.sub.1 /h.sub.2 is equal to or more than 0.1 and equal to or less than 500, preferably equal to or more than 0.15 and equal to or less than 50, and more preferably equal to or more than 0.5 and equal to or less than 21. When the wavelength of the surface acoustic wave is .lambda., h.sub.1 /.lambda. is 1 or less and h.sub.2 /.lambda. is 1 or less, preferably h.sub.1 /.lambda. is 0.5 or less and h.sub.2 /.lambda. is 0.4 or less, more preferably h.sub.1 /.lambda. is 0.25 or less and h.sub.2 /.lambda. is 0.25 or less.
The multilayer piezoelectric substrate 20 of a large electromechanical coupling coefficient of the present invention is preferably used in order to improve characteristics of a surface acoustic wave element of not only a surface acoustic wave amplifier and an acoustic surface convolver but also a surface acoustic wave filter, a surface acoustic wave resonator, etc.
As the active layer which constitutes the semiconductor layer of the present invention, one having a large electron mobility is preferably used. Preferred examples of the semiconductor film which constitutes the active layer include GaAs, InSb, InAs, and PbTe. Not only binary compound semiconductors but also ternary and quaternary mixed crystals derived from a combination of these binary semiconductors are preferably used. For example, ternary mixed crystals are In.sub.x Ga.sub.l-x As, InAs.sub.y Sb.sub.l-y, In.sub.z Ga.sub.l-z Sb and quaternary mixed crystals are In.sub.x Ga.sub.l-x As.sub.y Sb.sub.l-y etc. In-containing semiconductor thin films such as those made of InAs, InSb, InAsSb, InGaSb, InGaAsSb, etc. are used preferably since they have very large electron mobilities. Moreover, the active layer may be a multilayer film formed by stacking semiconductor films of different compositions. The electron mobility of the active layer is preferably 5,000 cm.sup.2 /Vsec or more so as to have a large amplification gain of the surface acoustic wave amplifier, and more preferably, 10,000 cm.sup.2 /Vsec or more so as to have a very good amplification gain. In order to obtain this large electron mobility, the active layer has a composition In.sub.x Ga.sub.1-x As where "x" can range 0.ltoreq.x.ltoreq.1.0, preferably 0.5.ltoreq.x.ltoreq.1.0, and more preferably, 0.8.ltoreq.x.ltoreq.1.0. The large electron mobility can be obtained when "y" of InAs.sub.y sb.sub.1-y ranges 0.ltoreq.y.ltoreq.1.0, and more preferably 0.5.ltoreq.y.ltoreq.1.0. "z" of In.sub.z Ga.sub.1-z Sb preferably ranges 0.5.ltoreq.z.ltoreq.1.0, and more preferably, 0.8.ltoreq.z.ltoreq.1.0.
Moreover, when a film thickness, h, of the active layer is 5 nm or less, its crystal characteristics is deteriorated and large electron mobility cannot be obtained. On the other hand, when h is 500 nm or more, the resistance of the active layer is lowered and at the same time the interaction efficiency of a surface acoustic wave and carriers is decreased. That is, in order to achieve large electron mobility and to perform the interaction of a surface acoustic wave and carriers efficiently, it is necessary that film thickness, h, of the active layer ranges 5 nm.ltoreq.h.ltoreq.500 nm, preferably 10 nm.ltoreq.h.ltoreq.350 nm, and more preferably 12 nm.ltoreq.h.ltoreq.200 nm. Moreover, a sheet resistance value of the active layer is preferably 10.OMEGA. or more, more preferably 50.OMEGA. or more, and most preferably 100.OMEGA. or more.
It is preferable that the buffer layer formed on the piezoelectric substrate of the present invention be insulated or semi-insulated. However, a large resistance value is available. For example, the resistance value of the buffer layer is at least 5 to 10 or more times as large as that of the active layer, preferably 100 times or more and more preferably 1000 times or more are preferable examples.
As the large resistance buffer layer, for example, a binary crystal such as GaSb, AlSb, ZnTe, or CdTe is preferably used. Ternary crystals such as AlGaSb, AlAsSb, GaAsSb, and AlInSb are used preferably. Quaternary crystals such as AlGaAsSb, AlInGaSb, AlInAsSb, AlInPSb, and AIGaPSb are used preferably. Moreover, when the composition of the buffer layer is determined, the lattice constant of the buffer layer is adjusted such that it has the same as or similar to the lattice constant of the crystal constituting the active layer. Thus, large electron mobility of the active layer can be achieved. Here, when the difference of the lattice constant of the crystals of the active layer and that of the buffer layer is .+-.7% or less, preferably .+-.5% or less and more preferably .+-.2% or less, the both lattice constants are similar to each other.
Further, during the actual step of forming the buffer layer 2 on the piezoelectric substrate 1, the lattice relaxation takes place extremely rapidly, particularly, in a buffer layer containing Sb. Even if its lattice mismatch with the piezoelectric substrate 1 is great, the lattice disorder is relaxed merely by growing an ultra-thin film of the buffer layer, and the buffer layer 2 begins to grow at a unique lattice constant specific to the crystal which constitutes the buffer layer 2. Immediately before the growth of the active layer, the buffer layer surface is in extremely satisfactory conditions, thereby greatly improving the crystallinity of the active layer 3 formed on the buffer layer 2. For this reason, an Sb-containing compound semiconductor is especially suitable for use as the buffer layer 2.
The thicker the buffer layer 2, the better its crystallinity. However, it is preferred the buffer layer 2 is as thin as possible from the standpoint of facilitation of the interaction between the surface acoustic wave and the carrier. More specifically, a preferred film thickness h.sub.3 of the buffer layer is 10 nm.ltoreq.h.sub.3.ltoreq.1,000 nm, and preferably, 20 nm.ltoreq.h.sub.3.ltoreq.500 nm.
Further, since the buffer layer 2 can grow at low temperatures, it is possible not only to prevent the piezoelectric substrate 1 from deteriorating due to leakage of oxygen, but also to prevent the active layer 3 formed on the buffer layer 2 from deteriorating due to migration of oxygen from the piezoelectric substrate 1. Further, the buffer layer 2 of the present invention is remarkably featured in that it serves as a protective layer for protecting the piezoelectric substrate 1 and the active layer 3, thus eliminating the need for the provision of a protective layer in the form of a dielectric film composed of SiO or SiO.sub.2.
Instead, no problem arises even if a dielectric film is present between the piezoelectric substrate 1 and the buffer layer 2. Examples of materials used for the dielectric film are SiO, SiO.sub.2, silicon nitride, CeO.sub.2, CaF.sub.2, BaF.sub.2, SrF.sub.2, TiO.sub.2, Y.sub.2 O.sub.3, ZrO.sub.2, MgO, and Al.sub.2 O.sub.3. The dielectric film is as thin as possible. Preferably, the thickness of the dielectric film is 100 nm or less, and more preferably, 50 nm or less.
The buffer layer 2 of the present invention, as compared with a dielectric film 9 such as SiO inserted between the piezoelectric substrate 1 and the active layer 3' of a conventional monolithic type amplifier, is lattice-matched to the active layer and has a large dielectric constant unexpected for semiconductors and a high resistivity. Accordingly, the electric field of the surface acoustic wave attenuates much less in the buffer layer 2 of the present invention. Therefore, the interaction between the electric field of the surface acoustic wave and the carriers in the active layer 3 occurs more efficiently than conventionally so that the buffer layer 2 of the present invention can be made thicker than the conventional dielectric film 9.
Further, a dielectric film or a semiconductor film may be grown on the active layer 3 as a protective layer in order to protect the active layer 3. As the dielectric film, the compositions indicated above may be used. As the semiconductor film, the same composition as that of the buffer layer may be used.
Generally, any method may be employed for forming films such as the buffer layer 2 and the active layer 3 as far as it allows growth of a thin film. It is especially preferred to employ a molecular beam epitaxy (MBE) method, a metal organic molecular beam epitaxy (MOMBE) method, a metal organic chemical vapor depposition (MOCVD) method, and an atomic layer epitaxy (ALE) method.
Further, in the present invention, the input and output electrodes 4 and 5 on the piezoelectric substrate 1 are electrodes having an interdigital structure. For such electrodes, an apodised transducer, a withdrawal weighted transducer, a unidirectional transducer, a normal type transducer and the like can be used. Particularly, the unidirectional transducer can reduce a loss due to bi-directional property of the surface acoustic wave. Consequently, the unidirectional transducer is used most preferably. Though materials for an input electrode 4 and an output electrode 5 are not particularly limited, it is preferred to use Al, Au, Pt, Cu, an Al-Ti alloy, an Al-Cu alloy, an Al and Ti multilayer electrode, for example.
In the case where the buffer layer 2 is formed such that the input and output electrodes 4 and 5 are embedded, the input and output electrodes 4 and 5 are formed in the first step. Thus eliminating the need to consider a recess and a projection of the semiconductor thin film allows the ultra-fine processing of the electrodes of the order of submicrons or less by the contact exposure.
However, since the input and output electrodes 4 and 5 are embedded in the buffer layer 2, it is necessary to select electrode materials which deform, melt, or diffuse in the least degree during the process of the formation of the buffer layer. Preferred examples of the electrode materials include Pt, Au, Cu, Al, Cr, Mo, Ni, Ta, Ti, W, and the like. Further, it is also preferred to use a multi-layered electrode of Ti-Pt, Ti-Al, Ti-Au, Cr-Au, Cr-Pt, etc.
There is no specific limitation to materials used as electrodes 6 for applying the DC electric field to the semiconductor layer in the surface acoustic wave amplifier of the present invention. Preferred examples of the electrode materials include Al, Au, Ni/Au, Ti/Au, Cu/Ni/Au, AuGe/Ni/Au, and the like.
When the surface acoustic wave functional element is used for a portion of a power amplifier which is required to have the capability of high power withstanding property, input and output electrodes 4 and 5 must use electrode materials which can withstand large power, i.e. power of an order of several Watts. Examples of preferred high power resistant materials for such electrodes capable of withstanding large power include an epitaxial Al film, an Al-Cu film, and Al-Cu/Cu/Al-Cu multi-layered film, a Ti-added-Al film, a Cu-added-Al film, a Pd-added-Al film.
The surface acoustic wave amplifier of the present invention may have the structure in which at least two separate semiconductor layers are successively formed on the substrate side by side between the input and output electrodes 4 and 5; and in addition, there is provided the arrangement which removes carriers moving in the direction opposite to the propagation direction of a surface acoustic wave or an arrangement in which the active layer 3 between the semiconductor layers is removed. With such a structure, the present invention can attain a high amplification gain at low voltages. For example, as shown in FIG. 6, semiconductor layers are separated by a mesa etching technique, or after the mesa etching process, a dielectric substance (not shown) is filled between the semiconductor layers so that the carriers moving in the opposite direction due to a reverse electric field can be removed.
In the surface acoustic wave convolver of the present invention, the electrodes formed on the piezoelectric substrate are used as two input electrodes. Further, the signal after convolution of the surface acoustic wave is taken out from take-out electrodes formed on an upper portion of the semiconductor layer and on a lower portion of the piezoelectric substrate, respectively. Materials for the take-out electrodes are not limited specifically. Preferably, Al, Au, Pt, Cu, and the like are used as take-out electrode materials.
In a conventional portable telephone set schematically shown in FIG. 7, to an antenna 10 is connected a duplexer 11, which is connected to a receiving amplifier 12 and a transmitting amplifier 13, each of the receiving and transmitting amplifiers 12 and 13 connected to a band pass filter 14. In contrast, as shown in FIG. 8, when the surface acoustic wave functional element capable of amplifying at a high amplification gain according to the present invention is applied to an RF portion, only a surface acoustic wave amplifier 15 for receiving and a surface acoustic wave amplifier 16 for transmitting are connected to the antenna 10. Therefore, according to the present invention, the number of component parts of the RF portion can be reduced as shown in FIG. 8, and each of the component parts can be made compact, lightweight and thin. Namely, the present invention can provide compact, lightweight terminals of portable appliances at low costs.